Capacitance techniques are commonly employed by distance measuring apparatus to inspect cavities and surfaces of conductive bodies for irregularities and imperfections. Typical capacitance-type distance measuring apparatus have two parts: a capacitance-type sensing probe; and, an electronic control system. The capacitance-type sensing probe includes electrodes that, in conjunction with the surface of a body being inspected, form capacitors that are used to make distance measurements. The electronic control system usually contains circuitry for applying charging voltages to the electrodes in the sensing probe and circuitry for translating capacitance-related signals into distance values. More specifically, a capacitance-type distance measuring apparatus measures the capacitance between the probe and the body being inspected. Once the capacitance is determined, the electronic control system translates the capacitance into a distance value. A capacitance-to-distance translation is relatively simple and straightforward and is based upon the inverse relationship that exists between capacitance and the distance between the plates that form a capacitor. Various physical characteristics of a body, such as size and shape can be determined from a suitable number of distance values.
A capacitance-type distance measuring apparatus that has found widespread use in the aircraft industry uses a parallel plate capacitance technique to inspect aircraft parts. However, another technique known as a fringe field capacitance technique offers certain advantages over the plate capacitance technique. These advantages, which include smaller sensing probes and more precise measurements, allow fringe field capacitance probes to be used to determine whether small cavities in aircraft parts are within exacting engineering specifications.
Capacitance values associated with fringe field capacitance probes are typically quite small--such as one-tenth of a picofarad. Accordingly, the electronic control systems of distance measuring apparatus must be sensitive to capacitance-related signals that are also quite small. The electronic control systems of prior art distance measuring apparatus have used methods such as circuit resonance, diode twin-T balancing, frequency modulation, phase-locked loop, and pulse width modulation to measure the capacitance between a probe and a conductive body being inspected. All of these prior art methods have inherent problems. For example, the circuit resonance method of capacitance measurement is most effective for signals within a narrow bandwidth. The narrow bandwidth limits the range of capacitance values that can be measured. Additionally, if a digital output signal is required, the circuit resonance method requires a complex analog-to-digital conversion. Diode twin-T circuits require substantial signal amplification that may introduce noise or other extraneous signals into the signal being measured. In addition, variations between the operating characteristics of diodes may require ongoing calibration of diode twin-T circuits. Frequency modulation and pulse width modulation methods experience oscillator stability problems and, when a digital output signal is required, involve complex analog-to-digital signal conversions. Phase-locked loop circuits behave nonlinearly over a wide range of capacitance values and are not well suited for measuring low values of capacitance, such as those created by fringe field capacitance probes.
As will be appreciated from the foregoing discussion, there has developed a need in the aircraft industry, as well as in other industries, for a method and apparatus that will accurately, reliably, and inexpensively measure the distance between a capacitance-type sensing probe, such as a fringe field capacitance probe, and a conductive body. The present invention provides a method and apparatus that computes the charge times of capacitors formed by the sensing probe and a nearby surface of the body in a manner that achieves these results.